Here are some suggestions for you.
MX licensing can now support mixed licensing in Co-termination licensing model with Per Device SD-WAN+ licenses, so, it's important to note that once you switch to the per-device licensing model, you cannot change back to the co-termination licensing model.
When moving devices between organizations, the static IP addressing set on the devices will change when moved to a different organization. The dashboard configuration that is applied to the device will not migrate to the new organization. It is recommended to pre-configure the receiving network to match the existing network before moving the device.
As you mentioned, Meraki requires VPN setups between Organizations to be created as "Non-Meraki VPN peers" even if they are both Meraki MX devices. This is because all MX security appliances within the same organization will be able to use the AutoVPN feature to establish a Site-to-site VPN between themselves. However, if two MX Security Appliances are in separate organizations, they will not be able to set up an automatic VPN.
For the issue of dynamic public IPs, it seems there's no straightforward solution. One user suggested using a DDNS service, but this might not be ideal.
One suggestion is to have a new subnet (VLAN) for your new devices behind the new MX. You can keep your original subnet behind the original MX and create a new transit VLAN that both MX's share. Then add routes in both MX's to allow the original subnet and the new subnet to communicate with each other.
Please note that these are suggestions. It's always best to consult with a network professional or Meraki support for advice tailored to your specific situation.
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
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